As adhesive or coating agents, those which have various kinds of curing systems are known. Among those, ones that can form a high crosslinking density adhesive layer or coated film by carrying out a curing reaction under heating, UV irradiation, and the like have recently received attention due to their possibility of exhibiting good adhesion strength or good strength of a coated film.
As a resin composition that can be cured by heating or UV irradiation, a radical polymerizable composition and an ionic polymerizable composition are mainly known.
Among those, as a radical polymerizable resin composition, for example, those containing a compound having a polymerizable unsaturated double bond and a radical polymerization initiator have been previously known.
However, there have been cases where the radical polymerization of the resin composition is terminated by a termination reaction between radicals or radical deactivation due to an effect of oxygen in the atmosphere before achieving sufficient curing of the composition. In addition, there have been cases where cure shrinkage of the radical polymerizable resin composition occurs during the polymerization reaction.
On the other hand, as an ionic polymerizable resin composition, cation-polymerizable ones are known in which an active species for polymerization is a cation, and anion polymerizable ones are known in which an active species is an anion.
Because the ionic polymerization does not employ a curing reaction by radicals, the ionic polymerization does not cause a radical termination reaction or radical deactivation as mentioned above and continues even in the case where UV irradiation or heating on the composition is ceased during the polymerization. In addition, the ionic polymerizable resin composition shows a lesser degree of cure shrinkage compared with a radical polymerizable composition.
Accordingly, the ionic polymerizable resin composition has been anticipated as a material applicable for optical elements such as digital video discs (DVDs) and polarizing plates in which a slight deformation of a substrate and the like may be a problem.
Regarding the ionic polymerizable resin composition, many kinds of compositions have been studied so far, and, for example, it has been found that an adhesive composition containing a cation-polymerizable compound, a compound having at least two hydroxyl groups in its molecule and a photo-cation polymerization initiator can be cured within a short period and has excellent properties such as adhesive strength, thus it can be used for manufacturing a liquid display panel and the like (for example, refer to Japanese Unexamined Patent Application, First Publication No. H10-330717). More specifically, the above Japanese Unexamined Patent Application, First Publication No. H10-330717 discloses an adhesive composition containing bis(2,3-epoxycyclohexyl methyladipate) as a cation-polymerizable compound, polytetramethyleneglycol as a compound having at least two hydroxyl groups within its molecule, and 7-[di(4-fluoro)phenylsulfonio]-2-chlorothioxanthone hexafluorophosphate as a photo-cation polymerization initiator.
However, since it is difficult to say that the above-mentioned adhesive composition has a practically sufficient level of adhesive strength for various substrates, and then in industrial fields there has been a demand for an adhesive exhibiting more excellent adhesive strength.
In addition, as the cation-polymerizable resin composition, for example, a curable composition containing at least one oxetane compound, at least one multifunctional alicyclic epoxy compound, at least one multifunctional hydroxy compound and at least one curing agent is known that can be used for various uses including an adhesive agent due to its fast curing rate after UV irradiation (for example, refer to Published Japanese Translation No. 2003-509553).
The Published Japanese Translation No. 2003-509553 discloses that as the multifunctional hydroxyl compound contained in the curable composition, aliphatic dihydroxy compound reactants such as higher polyoxyethylene glycols or higher polyoxytetramethylene glycols, aliphatic trihydroxy compound reactants such as higher polyoxypropylene glycols, or aliphatic multifunctional hydroxy compound reactants such as dendritic polymers which are commercially available as BOLTORN, a registered trademark from Perstorp Co. can be used.
However, there have been cases where a practically required level of adhesive strength cannot be achieved in the above-mentioned curable composition containing the aliphatic dihydroxy compound reactants or aliphatic trihydroxy compound reactants and the like as the multifunctional hydroxyl compound.
Meanwhile, there have been cases where a rather good adhesive strength can be achieved if the curable composition includes a polyester type dendritic polymer exemplified as the aliphatic multifunctional hydroxy compound reactants, as in the Published Japanese Translation No. 2003-509553.
However, it is difficult to say that the curable composition has excellent storage stability as a significant increase in viscosity may occur when stored for about 1 to 2 weeks or more. In addition, it is also difficult to determine whether the curable composition has a practically sufficient level of curability as it shows a fast curing rate, but if the obtained cured product is in contact with an organic solvent and the like, the cured product is partially eluted.
An object of the present invention is to provide a cation-polymerizable resin composition which can form a cured product which is excellent in storage stability, adhesive strength and curability.